Additive manufacturing processes, such as selective laser sintering and direct metal laser sintering, are used for applications such as prototyping and limited quantity production runs. Some of the benefits of additive manufacturing processes include the ability to produce highly complex parts quickly and efficiently, and to modify design specifications of the desired part, for example by modifying CAD specifications, without re-tooling casting or machining equipment used for traditional, subtractive manufacturing processes.
Laser sintering is utilized in many additive manufacturing processes. In a laser sintering process, a layer of pulverant material is applied to a work stage, and the layer is sintered using a laser into a solid or semi-solid sheet. Additional layers of pulverant material are applied over the work stage, and sintered to the underlying layer. In this way, a complex three-dimensional part is built up layer-by-layer.
Laser sintering operations are typically performed near the solidus temperature of the material used in the additive manufacturing process. The powder is heated by the laser to sinter (or sometimes melt) and combined with adjacent material. As soon as sintering is complete, the sintered portion is cooled below the melting temperature. In order to rapidly manufacture a part, the laser heats those portions that are to be solidified into the part as quickly as possible. This rapid heating and cooling can cause residual thermal stresses.
Residual thermal stresses generated during laser powder bed fusion not only affect geometrical accuracy of the components, but often cause process interruptions. The cumulative effect of thermal stresses in layered manufacturing can lead to distortion of parts, especially those parts having thin features. The components must be adequately anchored to the build plate to avoid excessive distortion. In extreme cases, the cumulative effect of thermal stresses can cause the powder recoater to stall on top of a previously solidified layer which has delaminated and jutted above the working stage. In other cases, components may crack due to internal stress, causing unwanted internal voids in the manufactured part. In both of these situations, the additive manufacturing process must be interrupted and the problem resolved before the part can be finished. As a result of this type of process interruption, the oxygen level in the additive manufacturing station may rise above an acceptable level, causing surface oxidation and lack of fusion to the subsequent layer.